Interleaving optical filter

ABSTRACT

An optical signal filter for providing a periodic transfer function in transmitting signals within a selected bandwidth, by which passbands are interleavered into groups of separate outputs. The filter employs the transmissivity characteristic of birefringent crystals in conjunction with splitting the input beam into orthogonal and separate components, while compensating for temperature variations by pairing crystals of different types. The transmissivity functions are independent of the polarization of the input beam, and are shaped to flatten transmissivity peaks by the use of cascaded stages of birefringent crystal pairs.

FIELD OF THE INVENTION

[0001] This application is a division-of U.S. application Ser. No.09/518,218, filed Mar. 3, 2000, now U.S. Pat. No. ______ issued ______2002.

[0002] This invention relates to wavelength division mutiplexed (DWDM)systems used in optical fiber communications, and more particularly tooptical signal filters which separate a WDM channel stream into groupsof channels on separate fibers.

BACKGROUND OF THE INVENTION

[0003] Narrow band optical filters are essential in wavelength division(WDM) communication systems in order to process signals at differentprecisely spaced wavelengths. Low insertion loss, flat top filterresponse, sharp cutoff, and the ability to scale to high channel countsand dense channel spacing are all critical parameters. An interleavingfilter is a device or subsystem which can separate multiple channels ina WDM transmission into groups. A 1×2 interleaving filter divides a WDMchannel stream, periodically spaced in optical frequency, in a mannersuch that every other channel is launched into one of two separatefibers. More generally a 1×N interleaving filter, separates every Nthchannel into one of N fibers.

[0004] The interleaving function, more broadly speaking, includesestablishing a periodic transmissivity characteristic within a givenwider frequency band, so that there is virtually lossless transmissionwithin incrementally spaced frequency channels, and in effect fullsignal rejection between the channels. Preferably, the transmissive passbands are shaped with flat top response, so that laser wavelength shiftsand other variations within the pass bands can be tolerated, thusreducing the stringency of performance specifications imposed on suchactive elements. Therefore, in multiplexing, channel spacings can bereduced with improved performance, while in demultiplexing closelyspaced channels can be separated without requiring prohibitively preciseindividual components, such as add/drop filters. In demultiplexing,interleaving filters can also serve to reduce the component counts andserial insertion losses, because they separate signals in parallelfashion and can be cascaded to divide channels into a number of smallergroups before wavelength selective devices are used to add or dropindividual wavelength signals.

[0005] The most common approach to interleaving filter design is basedupon using unbalanced Mach-Zender interferometers. These are adequatelyresponsive but are large, costly units that are difficult to adapt tomany system requirements. In addition, they are subject to inherentinstability problems that require extra measures to overcome. Thin film200 GHz filters are now being offered, but thin films require costly andprecise processes. Other periodic optical transmission functions areknown, such as those exhibited by birefringent crystals, as delineatedin detail by Yeh and Yariv in “Optical Waves in Crystals”, John Wileyand Sons (1983). As the authors explain, a birefringent elementsandwiched between parallel polarizers has a transmission characteristicthat is periodic in optical frequency, and effectively without loss attransmissive peaks. Much analytical work, of both theoretical andpractical natures has been directed to using the properties ofbirefringent crystals. In 1964, for example, Harris et al proposed aprocedure for the synthesis of optical networks in an article in theJournal of the Optical Society of America, Vol. 54, No. 10 (October1964), pp. 1267-1279, entitled “Optical Network Synthesis UsingBirefringent Crystals”. This article treats some of the considerationsfundamental to synthesizing specific transfer functions using a seriesof birefringent crystals between entry and exit polarizers.Subsequently, Kimura et al discussed a technique for reducing thermallyinduced variations in an article entitled “Temperature Compensation ofBirefringent Optical Filters”, in the Proceedings of the IEEE, August1971, pp. 1271-2. They disclosed that if the signs of the birefringenceof two different crystal are opposite, the retardation of the series isless dependent on temperture. Although the intended purpose of thedevice described is as a filter for frequency stabilization, one of thearticles cited, “Wide-band Optical Communication Systems, Part I—TimeDivision Multiplexing”, by T. S. Kinsel, Proc. IEEE, Vol. 58, October1970, pp. 1666-1683 is referenced in regard to the use of birefringentoptical filters to multiplex or demultiplex carriers of differentfrequencies in the field of wide-band optical communications.

[0006] A usage of crystals that is somewhat more related to theinterleaving filter context is disclosed in a letter published inElectronics Letters, Vol. 23, No. 3 dated Jan. 29, 1987, at pp. 106 and107, by W. J. Carlsen et al, discussing the use of a series ofbirefringent crystals configured to improve the characteristics ofsystems disclosed by articles on prior tunablemultiplexers/demultiplexers (referenced therein). All of thesemultiplexers are intended to be used with either of two lasers about 15to 25 nm apart in optical wavelength, but they do not suggest featuressuitable for an interleaving function or operation at the now common 100to 200 GHz spacings. A 100 GHz interleaving filter, for example,requires a passband of the order of 0.2 nm (vs. about 10 for the Carlsenet al system) and like intermediate stop bands. Carlsen et al do discussa modification which achieves a flattened passband using fiveretardation plates of selected orientations relative to the endpolarizers, and achieving polarization independence by splitting thebeam so as to direct polarization components separately through thefilter.

[0007] A need thus exists for a wideband interleaving filter havingmultiple narrow channel spacings and functioning with wide and flattenedpassband characteristics, insensitivity to polarization, temperaturestabilization and very low insertion loss. The need includes aconfiguration made of readily available materials that can be readilyassembled with the necessary precision, and that is of compact size andalso mechanically stable.

SUMMARY OF THE INVENTION

[0008] Interleaving circuits for optical networks in accordance with theinvention utilize a series of birefringent crystals in varyingelectrooptic property combinations and orientations to provide denselypacked periodic transmission peaks which nonetheless have very lowinsertion loss, polarization independence, flattened passband peaks andtemperature compensation. Pairs of dissimilar birefringent elements incascaded (series) relationship broaden the transmissive peaks whilecompensating out the effects of temperature variations. By mounting theelements on a planar reference structure having preset recesses in whichadjustments can be made, the unit can be aligned and adjusted withrespect to retardation, spacings and orientation for best performance.

[0009] In a more specific example of an interleaving filter inaccordance with the invention, birefringent crystals are arranged inseries between an input and output beam displacing polarizers, togetherwith beam combining elements at the output. The input beam is dividedinto two beams of orthogonal polarization, which are successivelyincident on two stages of paired birefringent crystals, the crystals ofeach pair being of opposite sign of thermooptic coefficient and ofspecific length ratios, and the crystals of the second pair being twicethe length of the first. With crystals of yttrium orthovandate (YVO₄)and lithium niobate (LiNbO₃), respectfully, the ratio used is 6.60 to 1,and the crystals are precisely spaced apart and provided withanti-reflection coatings on the beam-incident surfaces. The lengths usedare inversely related to the desired channel spacing. The optical (c)axes of the crystals are angled relative to the polarized input signalsand to each other to utilize the retardation difference of thebirefringent crystals, providing two temperature compensated outputbeams having flatband maxima, which are then split into another set inan output beam splitting polarizer. One combined beam of bothpolarization components is collimated for direction to one output fiber,while two separate beams of orthogonal polarization are combined in agroup of prisms and a polarizing beam splitter cube, for directionthrough a collimator to a second output fiber.

[0010] This interleaving filter, enclosed is a sealed housing is lessthan 20 cm long and 5 cm wide. Placement and angular orientation of theoptical elements is facilitated by the shaping of receiving recesses inthe optical bench, along the optical beam path. Where a pyroelectriccrystal such as LiNbO₃ is used, buildup of surface changes due totemperature cycling is avoided by current conduction from the crystalfaces that do not receive the optical beams. To achieve precise tuning,crystal faces may be angled so that relative translation of beams withrespect to the crystals changes the path length within crystals.

[0011] In accordance with other features of the invention, beamdisplacing polarizers are used in recombination of beams which havepassed through the birefringent crystal system. Path length differencescan be equalized by employing a half wave plate in both beams or acompensating plate in one of the beams incident on the beam displacingpolarizer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] A better understanding of the invention can be had by referenceto the following description, taken in conjunction with the accompanyingdrawings, in which:

[0013]FIG. 1 is a somewhat simplified perspective view of an example ofan interleaving optical filter in accordance with the invention;

[0014]FIG. 2 is a schematic side view representation of the filter ofFIG. 1 within a housing;

[0015]FIG. 3 is a schematic perspective representation of theorientation of crystals and polarizers in the filter of FIGS. 1 and 2,and an optical bench on which they are mounted;

[0016]FIG. 4 is a simplified plan view of the filter of FIGS. 1-3,illustrating beam paths along the structure;

[0017]FIG. 5 is a waveform diagram showing the periodic characteristicof a 100 GHz interleaving filter with passband flattening;

[0018]FIG. 6 is a waveform diagram of characteristics of a series ofbirefringent crystals, showing passband flattening;

[0019]FIG. 7 is a simplified block diagram view showing further featuresas to interleaving filter design and construction;

[0020]FIG. 8 is a simplified block diagram view showing further featuresas to interleaving filter design and construction, and

[0021]FIG. 9. is a simplified block diagram view showing furtherfeatures as to interleaving filter design and construction.

DETAILED DESCRIPTION

[0022] Birefringent or polarization filters are described in “OpticalWaves in Crystals”, Yeh and Yariv, referenced above. The transmissionthrough a birefringent element sandwiched between parallel polarizers isperiodic in frequency and is given by: $\begin{matrix}{{I(\lambda)}:=\left\lbrack {\cos \left\lbrack {\pi \cdot \frac{\left( {n_{e} - n_{0}} \right)}{\lambda} \cdot L \cdot 10^{6}} \right\rbrack} \right\rbrack^{2}} & {{Eqn}.\quad 1}\end{matrix}$

[0023] where I is the intensity, λ is the wavelength in nanometers,n_(e) is the extraordinary index of refraction, n_(o) is the ordinaryindex of refraction and L is the thickness of the crystal in mm. Thecrystal is oriented with its optic axis at 45 degrees to the inputpolarization. Note that in this governing relationship the transmissionis periodic in optical frequency (i.e. inverse wavelength) and thetransmission is lossless at the peak. The present systems use thisfundamental controlling Eqn. 1 together with a number of others inproviding shaped transmissivity characteristics with low insertion loss,polarization insensitivity and compensation for temperature variation.

[0024] FIGS. 1 to 4 depict the arrangement and relationships of thecomponents of a 1×2 passband flattened and temperature compensatedinterleaving filter 10, the components of which are seated on agenerally planar surface 13 of a stainless steel optical bench 12 (seeFIGS. 2 and 3). The optical bench 12 (which alternatively can be ofother materials such as silicon) contains shaped recesses 14 formed byelectron discharge machining (EDM), in which recesses 14 in the planarsurface 13 the polarizing components and crystals are mounted andprecisely aligned and angled. A gradient index (GRIN) lens responsive tothe wideband input stream from an optical fiber is a collimator 16 forthe WDM channel stream. The collimator lens 16 is secured within acylindrical metal housing 20 which is welded within a stainless steelclip 22 of general U-shape, where base legs 24, 25 are welded to theoptical bench 12. The clip 22 design allows for precise and stable tiltand translation adjustment of the collimator lens during assembly.

[0025] After collimation, the input beam is transmitted through a beamdisplacing polarizer 28, here of YVO4 crystal, which splits the inputbeam into two parallel beams (FIG. 4 particularly) with crossedpolarizations that are shifted 1 mm with respect to the other. Splittingthe input beam into separate polarizations and then recombining afterfiltering results in a polarization independent device.

[0026] The two beams are then incident on a first birefringent crystalstage comprised of a Yttrium orthovanadate (YVO4) crystal 30 and aLithium niobate (LiNbO₃ crystal, 32 configured to form a first atemperature compensated pair. There are a number of differentbirefringent materials which can be used for temperature compensation,including YVO4 and LiNbO3. YVO4 has high birefringence, Δn=0.2039 at1550 nm, and is readably available commercially. LiNbO3 has a largethermooptic coefficient opposite in sign to YVO4 and is also readablyavailable commercially. The required length ratio for temperaturecompensation of YVO4 to LiNbO3 is 6.60:1. The lengths scale inverselywith channel spacing, and 50 Ghz and 25 GHz spacing are achievable. Theoptical (C) axis of the YVO4 crystal and the LiNbO3 crystal 32 areoriented at 45° relative to the input polarizer 28. For a 100 Ghz to 200Ghz channel spacing the YVO4 crystal 30 is 7.370 mm long and the lithiumniobate crystal 32 is 1.116 mm long.

[0027] The second stage, which is also temperature compensated butemployed to flatten the peak of maximum transmissivity of the passbandis comprised of another set of YVO4 36 and lithium niobate crystals 38.The lengths of each of these are twice that of the like crystals used inthe first stage. The optical axes of each separate crystal 36,38 of thesecond pair are oriented along a crystal edge and the crystals aretilted—14.8 degrees with respect to the top surface 13 of the opticalbench 13. The edges of the lithium niobate crystals 32 and 38 areelectrically shorted with a conductive coating 40 such as silver epoxywhich conducts away charges built up due to the pyroelectric property ofLiNbO₃. Such electric charges would otherwise tend to build up on thesurface of crystals 32, 38 as the temperature is cycled, and the resultcould be uncontrolled hysteresis of the index of refraction. Suchcharges must be eliminated for LiNbO₃ to be used as a temperaturecompensating element. Conductive silver epoxy 40 (or metallization oranti static liquid) of the +/− c faces of a crystal electrically shortsthe crystal and dissipates charges. Dopants such as MgO which reduce theelectrical conductivity would also reduce the pyroelectric effects.

[0028] The two beams after being transmitted through the birefringentcrystals 30, 32, 36, 38 are incident on another YVO4 beam displacingpolarizer 41. Each input beam is split up into two beams with differentpolarizations, as best seen in FIG. 4. Two of the beams with crossedpolarization overlap and are coupled directly into one of two outputGRIN lens collimators 47, 48. The two other beams are combined in partby using a single prism 42 to direct the s polarized beam to one side ofa polarizing beam splitter cube, 43 with the p polarized beam beingredirected off a pair of prisms 44, 45 to an orthogonal side of the beamsplitter cube 43. The path lengths of the two combined beams are matchedto better than 1mm in order to minimize polarization mode dispersion(PMD). The resulting overlapping beams are then coupled into the secondoutput collimator 48. The output collimators 47, 48 are laser welded toclips which are in turn laser welded to the optical bench 12.

[0029] Referring specifically to FIG. 2, the optical bench 12 is mountedinside a tray 50 with a fiber feedthrough 52 in the end wall 54receiving an input optical fiber 18 in line with the input collimator16. Although the output side is not shown in this view it differs inhaving only pairs of elements for delivering the two output beams. Thebench 12 is attached to the base of the tray 50 with RTV adhesive or, asshown, a silicone sheet 58 can be used to provide cushioning from shockand vibration. The input fiber 18 and two output fibers are fed throughthe fiber feedthroughs and sealed with epoxy. A lid 58 is attached tothe body of the tray 50 and the waist is sealed with epoxy in a drynitrogen atmosphere.

[0030] All of the optical surfaces, including the crystals, areantireflection coated to minimize optical loss.

[0031] The experimentally measured transmission (using an LED and anoptical spectrum analyzer) of a fiber coupled, passband flattened 100Ghz interleaver is shown in FIG. 5. The unit uses two stages of lithiumniobate and YVO4 crystals. This measured response is charted in FIG. 5.The spacing between transmissivity peaks is that of a 100 Ghz channelspacing to 200 Ghz channel spacing interleaving filter.

[0032] Wider passband flatness in a filter (i.e. a broadening of thewidth of the transmissivity peaks), prevents narrowing of thetransmission spectra when filters are cascaded and reduces the requiredwavelength accuracy of the WDM source lasers. It also improves systemperformance by reducing the attenuation of the information content of amodulated signal. By adding additional birefringent elements, thepassband of the interleaving filter is flattened to a selectable degree.As shown in FIG. 6, which depicts response variations between one, twoand three crystals in series, the passband of a single elementbirefringent filter is 0.35 nm wide at the −0.5 dB bandwidth By adding asecond and third birefringent element, shown by dotted and dashed linesrespectively, the maximum is progressively broadened. The first elementis of length L and oriented with its C axis at 45 degrees. For a seriesof two, the second element is of length 2L and has an orientationof—14.8 degrees, substantially widening the amplitude at maximum withoutbroadening the cutoff point. Addition of a third element of length 2Land orientation =+10 degrees broadens the maximum even further, butintroduces intermediate dips of minor magnitude. For a 100 Ghz/200 ghzinterleaver the bandwidth for a passband flatness of −0.5 dB is 0.35 nmfor the single stage, 0.47 nm for the two stage, and 0.60 nm for thethree stage design. Even better flatness can be achieved by adding moreelements; however, this comes at the expense of additional cost andinsertion loss. Although the curves depict the results of measurementswith crystals of only one type, they are equally valid for temperaturecompensated combinations using different crystal types.

[0033] The polarization dependent loss (PDL) of the interleaving filtermust be minimized to a value below 0.1 dB. This is achieved duringcoupling of the two crossed polarization beams incident on each outputGRIN lens collimator 46 or 48. In order to minimize PDL the beams mustbe coupled into the output fiber with the same efficiency. This is notnecessarily at the peak coupling efficiency of each beam. This can bedetermined by varying or switching the input polarization to theinterleaving filter 10 until no variation of power on the output fibersis measured.

[0034] In order to make a polarization independent fiber basedinterleaving filter, the signal is split up into two beams using thelossless beam displacing polarizer 28. The two beams are transmittedthrough the birefringent elements 30, 32, 36, 38 and recombined into twooutputs using the additional lossless beam displacing polarizer 42. Thes polarized output of one of the beams is recombined with the ppolarization of the other beam. Every other channel of a WDM stream isthus separated into one of the two output collimators 47, 48 and theoutput fibers to which they couple.

[0035] Devices typically operate over a 0-70° C. temperature range andshould be passively temperature compensated. For a 100 Ghz filterresponse the center wavelength drift should typically be less than+/−0.0015 nm/° C. The use of different crystals with opposite signs ofthe birefringence or thermooptic coefficient in the manner describedachieves this result. The retardance of two crystals in series is givenby:${\Gamma_{1} + \Gamma_{2}}:={\frac{2 \cdot \pi}{\lambda} \cdot \left( {{{L_{1} \cdot \Delta}\quad n_{1}} + {{L_{2} \cdot \Delta}\quad n_{2}}} \right)}$

[0036] Where Γ is the phase retardance, L is the length of each crystal,and the birefringence is given by Δn₁=n_(e,1)−n_(o,1) for the firstcrystal. For the second crystal Δn₂=n_(e)−n_(o) if the crystal axis isparallel to that of the first crystal and is Δn₂=n_(o)−n_(e) if it isrotated 90 degrees. The change of retardance with temperature is givenby:$\frac{{\left( {\Gamma_{1} + \Gamma_{2}} \right)}\quad}{T}:={\left( \frac{2 \cdot \pi}{\lambda} \right) \cdot \left( {{L_{1} \cdot \frac{\left( {\Delta \quad n_{1}} \right)}{T}} + {L_{2} \cdot \frac{\left( {\Delta \quad n_{2}} \right)}{T}} + {{L_{1} \cdot \alpha_{1} \cdot \Delta}\quad n_{1}} + {{L_{2} \cdot \alpha_{2} \cdot \Delta}\quad n_{2}}} \right)}$

[0037] Where α is the thermal expansion coefficient. The condition forcompensation is given by:$\frac{L_{1}}{L_{2}}:=\frac{- \left( {\frac{\left( {\Delta \quad n_{1}} \right)}{T} + {{\alpha_{1} \cdot \Delta}\quad n_{1}}} \right)}{\left( {\frac{\left( {\Delta \quad n_{2}} \right)}{T} + {{\alpha_{2} \cdot \Delta}\quad n_{2}}} \right)}$

[0038] Usually the thermal expansion coefficient term can be neglected.With the optic axis of the crystals in alignment, compensation isachieved using two crystals with different signs of the thermoopticcoefficient. If the crystals are rotated 90 degrees with respect to eachother, materials can be used with the same sign of the thermoopticcoefficient.

[0039] During assembly, both the frequency period and absolutewavelength of the peaks must be adjusted. This can be controlled bytight tolerances of the thickness of the polished crystals. The crystalsor the input beam angle can also be tilted to adjust the wavelength.Another approach is to polish the crystal to form a slight wedge shape,with the beam-incident faces thus being non-orthogonal to beamdirection. Then the wavelength and period can be adjusted by translatingthe beam on the crystal. In order for both the parallel beams to see thesame thickness of crystal, the wedge angle should be transverse to theplane of the two incident beams. Two crystals with opposing wedges canbe translated relative to each other to adjust the thickness andminimize any beam steering. Another approach to tune the wavelength isto choose from a set of LiNbO3 crystals at slightly different thickness,and tuning by substituting for the best response. A spacing of 10microns can allow for tuning while only slightly changing thetemperature compensation condition. Adjustment of the absolutewavelength peaks of the filter can also be achieved by using a zeroorder half wave plate after each of the stages. By rotating thewaveplates additional birefringence is introduced which tunes thefilter. Zero order waveplates are used to minimize temperaturedependence of the waveplate.

[0040] The alignment and tolerance of the optical components arecritical. Both insertion loss and manufacturing assembly cost need to beminimized. An alternative to the stainless steel optical bench is to usea silicon bench as a platform to mount all of the components. PreciseV-grooves are etched onto the silicon substrate and components aredropped into them and attached with epoxy.

[0041] 1×2 interleavers can be cascaded to split every Nth channel intoone of N output fibers. For the second stage a crystal of half thethickness of the first stage is required. In general the transmissionthrough an N stage interleaver will have a transmittance at one of theoutput fibers given by:${I(\lambda)}:={\prod\limits_{i = 1}^{N}\quad {n\left\lbrack {\cos \left\lbrack {\pi \cdot \frac{\left( {n_{e} - n_{0}} \right)}{\lambda} \cdot \frac{L}{i} \cdot 10^{6}} \right\rbrack} \right\rbrack}^{2_{4}}}$

[0042] The other fibers will have the same wavelength dependenttransmittance with the peaks shifted by a multiple of the input channelspacing.

[0043] Each stage of a birefringent filter is not limited to separatingevery other channel. More generally a single stage can group every Nthchannel onto a single fiber and the remaining (N−1) adjacent channels ineach period onto a separate fiber. One approach is to use a Solc typefilter described in the Yeh and Yariv treatise referred to above. Thereare two designs, folded and fanned, which rely on a stack of rotatedbirefringent plates of equal thickness.

[0044] Another example of an arrangement in accordance with theinvention, referring now to FIG. 7, divides the input beam from an inputcollimater 60 into an s polarized beam and an orthogonal p polarizedbeam at a first beam splitting polarizer cube 62. The p polarized beamis directed back into parallelism for compactness at a prism 63, andboth beams then pass separately through sheet polarizers 64, 65 totemperature compensating birefringent crystal pairs 70, 71 and 73, 74 asdescribed above, after which separate beam pairs are recombined. Spolarized components are angled off a prism 80 to one face of a secondpolarizer cube 82, which receives the p polarized beam at another face.From the second cube 82 two orthogonal beams merge, each combining s andp components, and the two combined beams are directed to first andsecond collimators 84, 85 respectively. This arrangement simplifies beamrecombination but, because of the characteristics of polarizing beamsplitters, the cross-talk between adjacent channels is higher, eventhough reduced somewhat by the sheet polarizers. In addition, the costsof using separate crystals of equal lengths must be considered.

[0045] Other examples of arrangements in accordance with the invention,referring to FIG. 8 and FIG. 9 make use of different opticalarrangements to recombine the s and p polarized beams into the secondoutput fiber. In FIG. 8 the optical layout from the input collimatorthrough the birefringent crystals and the second beam displacingpolarizer is the same as described previously. Here, however, a prism 90is used to pick off the center beam emerging from the second beamdisplacer 41 which contains both required polarizations. This beam isreflected with another prism 92 and coupled directly into an outputcollimator 47. The other two beams emerging from the second beamdisplacer 41 are recombined within a third beam displacing polarizer 94.The length of the last beam displacer 90 is twice that of the first two28, 41 due to the need for twice the displacement. Since the pathlengths of the two beams would not otherwise be matched, a compensatingplate 96 is inserted in one of the beam paths to match the optical pathlength. A high index material such as lithium niobate, with the crystalaxis aligned with the input polarization, is used in the s polarizedbeam.

[0046] Another approach to match the optical path lengths is shown inFIG. 9. The two beams emerging from the second beam displacer 41 aretransmitted through a half wave plate 98 and then recombined using athird beam displacement polarizer 100. The half wave plate 98 rotatesthe polarization 90 degrees, which ensures that the overall path lengthsof the two beams are matched after going through the final beamdisplacer 100. The half wave plate 98 is a zero order design to reducethe temperature dependence. The combined beams in the midregion of thesecond beam displacing polarizer 41 are angled off a common prism 102 tothe second output collimator 48.

[0047] Although a number of variants and alternatives have beendescribed, the invention is not limited thereto but emcompasses allforms and modifications within the scope of the appended claims.

1. An interleaving optical filter for wave energy, for providing aperiodic low loss transmissivity characteristic in the rate of 25 GHz to200 GHz spacing and operating with substantial polarization independenceand with compensation for temperature variations, said filtercomprising: a support providing a generally planar surface extendingsubstantially parallel to a principal optical axis for the filter; aninput collimator mounted on the support at an input region thereon toprovide a collimated beam along the principal optical axis; a first beamdisplacing polarizer mounted on the support to receive the collimatedbeam, the polarizer transmitting two beams of orthogonal polarizationthat are parallel to the principal optical axis; a first pair ofbirefringent crystals receiving the two beams and being of differentthermooptic coefficients and with lengths along the principal opticalaxis that are selected to compensate for temperature-induced phaseretardation variations, the first pair being rotated 45° with respect tothe planar surface about the principal optical axis; a second pair ofbirefringent crystals of materials like the first pair but of differentlength, and being rotated with respect to the planar surface to providetransmissivity peaks that have passband flatness of −0.5 dB of about0.47 nm and a center wavelength drift of less than ±0.0015 nm/° C.; asecond beam displacing polarizer receiving the two beams transmittedthrough the pairs of birefringent crystals for splitting each of the twobeams into two beams with different polarizations; a beam recombiningunit receiving the beams from the beam displacing polarizer forcombining the beams therefrom into two polarization independent beams inthe component beams with less than one 1 mm path length difference. 2.An interleaving optical filter as set forth in claim 1 above, includingoutput collimators coupled to transmit the different polarizationindependent beams and an interleaving optical filter as set forth inclaim 1 above, wherein the second beam displacing polarizer transmits apair of orthogonally polarized individual beams and a combined beamhaving orthogonally polarized components, and the beam recombining meansdirects the combined beam as one of the outputs.
 3. An interleavingoptical filter as set forth in claim 1 above, wherein the second beamdisplacing polarizer transmits a pair of orthogonally polarizedindividual beams and a combined beam having orthogonally polarizedcomponents, and the beam recombining unit directs the combined beam asone of the outputs.
 4. An interleaving optical filter as set forth inclaim 3 above, wherein the beam recombining means includes a third beamdisplacing polarizer receiving the orthogonally polarized individualbeams, and further includes a path length compensator in one of the beampaths to the third beam displacing polarizer.
 5. An interleaving opticalfilter as set forth in claim 3 above, wherein the beam recombining meansincludes a third beam displacing polarizer receiving the orthogonallypolarized individual beams, and further includes a half wave plate inboth beam paths to the third beam displacing polarizer.
 6. Aninterleaving optical filter as set forth in claim 3 above, wherein thebeam recombining means comprises a polarizing beam splitter cube andprism means for directing the orthogonally polarized individual beams todifferent faces of the beam splitter cube.
 7. An interleaving opticalfilter as set forth in claim 1 above, wherein the birefringent crystalsare of opposite sign, and wherein the second pair of crystals have anegative angular rotation relative to the first pair.
 8. An interleavingoptical filter as set forth in claim 1 above, wherein the pairs ofbirefringent crystals each comprise a YV04 crystal and an LiNbO₃ crystalhaving length ratios of 6.60:1 and wherein the crystals of the secondpair are twice the length of those in the final pair.
 9. An interleaverfilter as set forth in claim 1 above, wherein the first and second beamdisplacing polarizers are of YV04 crystal and the beam recombining unitcomprises a prism and polarizing beam splitter cube.
 10. An interleavingoptical filter as set forth in claim 2 above, wherein the inputcollimator and output collimators comprise gradient index lenses,wherein the filter further includes housings attached to the collimatorsand the housings are attached to the support, wherein the collimatorsare disposed along the principal axis, and the filter further comprisesinput and output optical fibers in communication with the input andoutput collimators respectively.
 11. An optical assembly for retaining anumber of birefringent elements, polarizing elements and collimatingelements in precise axial and rotational positions along an opticalaxis, comprising: an optical bench having a principal planar surface,the surface including inset recesses disposed at spaced locations alongthe length of the optical axis; the recesses having angles relative tothe planar surface to define the rotational orientation of thebirefringent elements; adjustable attachment mechanisms attached to theoptical bench at collimator positions along the optical axis; collimatorhousings each supporting a different collimator and each attached to adifferent one of the attachment mechanisms; and a containment housingencompassing the optical assembly and including optical fiberfeedthroughs along the optical axis and in alignment with collimatingelements therein.
 12. An optical assembly in accordance with claim 11above, wherein the optical bench is of stainless steel, and wherein inaddition to the attachment mechanisms are laser welded to the opticalbench and at least one of the birefringent elements are conductivelycoupled to the optical bench.
 13. An optical assembly in accordance withclaim 12 above, wherein the housing includes a tray and a lid in asealed configuration, and a layer of resilient material supporting theoptical bench in the tray.
 14. An interleaving optical filter forintroducing a periodic transfer function with flattened apices withintransfer in a wider band spectrum of an input optical beam, comprising:a first polarizer in the path of the input optical beam; at least twostages of pairs of birefringent crystals of opposite thermoopticcoefficients, the crystals of each pair having a like lengthproportionality but the lengths of the crystals in the first pair havinga selected ratio to the lengths of the crystals of the second pair, thelonger pair being disposed closer to the first polarizer; and a secondpolarizer in the path of the optical beam subsequent to the two stages.15. An optical filter as set forth in claim 14 above, wherein the likecrystals within each stage have like optical axes and orientationsrelative to the polarizer direction and wherein the filter furtherincludes beam displacing means between the first polarizer and the atleast two stages for directing beams of orthogonal polarization throughthe at least two stages, and wherein the filter further includes anoptical circuit for recombining the beams of orthogonal polarizationinto two beam sets, each including both polarizations.
 16. An opticalfilter as set forth in claim 15 above, wherein the lengths of thecrystals are selected to provide a selected periodicity in the transferfunction, and wherein the optical circuit recombines the beams in thebeam sets with equal path lengths.
 17. An optical filter as set forth inclaim 16 above, wherein each pair of crystals comprise a YVO4 crystaland an LiNbO₃ crystal, and wherein the C axes of the first pair are at45° to the direction of the first polarizer and the C axes of the secondpair are at □14.8°, and wherein the length proportionality of thecrystals in each pair is 6.60:1 and the ratio between the two pairs is1:2.
 18. An Optical filter for introducing an interleaving function intoa signal beam occupying an optical band, comprising: a polarizing beamsplitting cube for dividing the signal beam into two orthogonallypolarized beams; first and second birefringent crystal sets, eachincluding an initial polarizer sheet and two pairs of serially disposedbirefringent crystals of opposite thermooptic sign; and beam recombiningmeans including a second polarizing beam splitting cube receiving thetwo orthogonally polarized beams and providing two output beams, eachincluding both polarization components.
 19. A filter for separating aninput band of optical signal frequencies into a number of periodicityvarying transmissive frequency bands divided into at least two groups,comprising: at least one pair of birefringent crystals disposed seriallyalong an optical axis for receiving the optical signals, at least one ofthe crystals having a pyroelectric characteristic; polarizing meansdisposed along the optical axis and bordering the at least one pair ofoptical crystals; and means coupled to crystals having a pyroelectriccharacteristic for dissipating electric charges induced thereon.
 20. Afilter as set forth in claim 19 above, wherein the crystals of a pairhave optical axes that are similarly aligned relative to the opticalaxis of the device, and wherein the polarizing means comprises a beamdisplacing polarizer for dividing the optical signals into two beamsdirected along the optical axis.
 21. A filter as set forth in claim 20above, wherein the at least one pair of birefringent crystals comprisestwo pairs, the second pair having alignments of their optical axes thatare alike within the pair but different from the other pair, and have aselected length relationship to the crystals of the other pair.
 22. Afilter as set forth in claim 19 above, wherein the means for dissipatingelectric charges comprises conductive coating material disposed onselected surfaces of the crystals.
 23. An optical filter fortransferring optical signals in either direction between a terminal atone side and a pair of terminals at the other side, in either direction,the optical signals periodically spaced in wavelength, the filtertransferring the signals with a selected periodic passband function,comprising: at least two serial stages of birefringent light propagatingelements arranged to provide differential retardations betweenorthogonal polarization components of the optical beams, each stagebeing configured with at least two elements of different thermoopticcharacteristics to be substantially athermal over a selected temperatureband, the differential retardation relationships being selectivelyvaried between the stages to provide selected passband widths at theselected passband periodicity; at least one polarization responsive beampath juncture device between the terminal at one side and the at leasttwo stages and transferring optical signals therebetween, splitting thesignals in the beam path in accordance with polarization in onedirection, and combining the signals transferred in the other direction;and at least two beam path juncture devices each between the terminal ofa different one of the pair of terminals and the at least two stages forprocessing optical signals therebetween, said beam path juncture devicesbeing polarization responsive and disposed in series, and includingelements arranged to cross-combine signals of orthogonal polarization,such that the filter is polarization insensitive.
 24. A filter as setforth in claim 23 above, wherein the light propagating elements arearranged in two sets of at least two stages disposed in separate paths,and like elements in the parallel paths are of matched lengths andthermooptic characteristics.
 25. A filter as set forth in claim 23above, wherein the filter further includes separate lens collimators atthe outputs of the separate ones of the at least two beam path juncturedevice, and means associated with the at least one beam path juncturedevice for establishing the input polarizations of the beams to minimizethe polarization dependent loss to below 0.1 dB.
 26. A filter as setforth in claim 23 above, wherein the relative angles of the fast axes ofthe stages are selected to vary the retardation relationships, andwherein the filter includes elements for equalizing the path lengths ofthe separate optical beams to minimize PMD.
 27. A filter as set forth inclaim 23 above wherein the stages have an athermal characteristic suchthat the center wavelength drift is less than 0.0015 nm/° C.
 28. Afilter as set forth in claim 23 above, wherein the athermal compensationconditions is approximately in accordance with:$\frac{L_{1}}{L_{2}}:=\frac{- \left( {\frac{\left( {\Delta \quad n_{1}} \right)}{T} + {{\alpha_{1} \cdot \Delta}\quad n_{1}}} \right)^{n}}{\left( {\frac{\left( {\Delta \quad n_{2}} \right)}{T} + {{\alpha_{2} \cdot \Delta}\quad n_{2}}} \right)}$

where L is the length of each crystal, Δn is the birefringence for acrystal, and α is the thermal expansion coefficient.
 29. Amultiplexer/demultiplexer functioning in accordance with an interleavertransfer function for processing interleaved, wavelength multiplexedsignals of a first inter-channel wavelength periodicity present at oneterminal and signals at half the first inter-channel wavelengthperiodicity present at a pair of terminals, comprising: a pair ofoptical beam paths, each comprising at least two stages of optical delayelements disposed in series, each stage of each path including at leasttwo optical elements in series, whose lengths and thermoopticcharacteristics are selected to provide a differential phase retardationbetween orthogonally polarized beam components that is athermal over aselected temperature range to define a periodic transmissioncharacteristic of chosen periodicity, the lengths and fast axisorientations of each stage being selected to broaden the passbands ofthe transmission characteristic; a first beam splitter/combiner in theoptical communication path between the one terminal and the pair of beampaths for (1) directing and separating received signals of arbitrarystate of polarization from the one terminal into the two beam paths, thesignals on the two beam paths being orthogonally polarized in a fixedrelation to one another, and for (2) combining orthogonally polarizedoptical beams from the beam paths into signals of arbitrary state ofpolarization at the said one terminal; and a second and third beamsplitter/combiner means in the optical communication path between thepair of beam paths and the pair of terminals, for (1) cross-combiningorthogonally polarized beam components received from the beam paths andexhibiting periodic polarization characteristics at half the firstwavelength periodicity into separate beams exhibiting periodictransmission characteristics at half the first wavelength periodicityand transferring them to the pair of terminals, and for (2) separatingsignals received at the pair of terminals, signals exhibiting periodictransmission characteristics at half the first periodicity, intoorthogonally polarized signals, and transferring orthogonally polarizedsignals to the beam paths through stages as individual beams.
 30. Amultiplexer/demultiplexer as set forth in claim 29 above, whereinmultiplexed signals applied to the terminals have arbitrary states ofpolarization and where the beam splitter/combiners are polarizationsensitive and divide or combine beams in accordance with polarizationand direction.